The present invention relates to transimpedance amplifiers, and more particularly to nested transimpedance amplifiers with an increased gain-bandwidth product.
A transimpedance amplifier (TIA) is a well-known type of electronic circuit. Referring now to FIG. 1, a TIA 100 includes an operational amplifier (opamp) 105 having a gain parameter (gm). The opamp 105 is connected in parallel to a resistor (Rf) 110. The input to the TIA 100 is a current (xcex94i) 115. The output of the TIA 100 is a voltage (xcex94vo) 120.
Referring now to FIG. 2, the opamp 105 of the TIA 100 is replaced by a current source 205 and a transistor 210 having gain gm. The TIA 100 in FIGS. 1 and 2 is often referred to as a transconductance amplifier because it converts the input current xcex94i into the output voltage xcex94vo.
Referring now to FIG. 3, a TIA 300 converts an input voltage (xcex94vi) 305 into an output voltage (xcex94vo) 310. The TIA 300 also includes a resistor 315 that is connected to a transistor 320. The TIA 300 is typically used in applications that require relatively low bandwidth.
Referring now to FIG. 4, a TIA 400 converts an input voltage (xcex94vi) 405 into an output voltage (xcex94vo) 410. The TIA 400 includes a second opamp 415, which is connected in series to a parallel combination of a resistor (Rf) 420 and an opamp 425. The TIA 400 is typically used for applications having higher bandwidth requirements than the TIA 300.
Ordinarily, the bandwidth of the TIA is limited to a fraction of a threshold frequency fT of transistor(s) that are used in the opamp(s). In the case of a bipolar junction transistor (BJT) such as a gallium-arsenide (GaAs) transistor, the bandwidth of the TIA is approximately equal to 10%-20% of fT. For metal-oxide-semiconductor (MOS) transistor(s), the bandwidth of the TIA is typically a few percent (i.e., approximately 2%-6%) of fT.
Referring now to FIG. 5, a TIA 500 may be configured to operate differentially using two inputs of each opamp 502 and 504. One input 505 acts as a reference, in a similar manner as ground or virtual ground in a standard configuration TIA. The input voltage xcex94vi and the output voltage xcex94vo are measured as voltage differences between a reference input 505 and a second input 510. Feedback resistors 514 and 516 are connected across the inputs and the outputs of the opamp 504.
Referring now to FIG. 6, one TIA application having a relatively high bandwidth requirement is that of an optical sensor. An optical sensor circuit 600 includes the opamp 105 and the resistor 110 of the TIA 100 that are coupled with a photodiode 605. The output of the photodiode 605 is a current Iphoto 610, which acts as an input to the TIA 100.
Increasingly, applications require both high bandwidth and high gain. Examples include optical sensors, such as fiber optic receivers, and preamplifier writers for high-speed hard disk drives. Efforts to increase the gain-bandwidth product of TIAs have been made. For example, in U.S. Pat. No. 6,114,913, which are hereby incorporated by reference, a boost current is used to increase the gain-bandwidth product in the TIA. Cascading TIA stages is also used in U.S. Pat. Nos. 5,345,073 and 4,772,859, which are hereby incorporated by reference.
Other improvements to TIAs are the subject of other patents, such as U.S. Pat. Nos. 6,084,478; 6,057,738; 6,037,841; 5,646,573; 5,532,471; 5,382,920; 5,010,588; 4,914,402; 4,764,732; 4,724,315; 4,564,818; and 4,535,233, which are hereby incorporated by reference. However, improving the gain-bandwidth product of TIAs continues to be a challenge for circuit designers.
A nested transimpedance amplifier (TIA) circuit according to the present invention includes a zero-order TIA having an input and an output. A first operational amplifier (opamp) has an input that communicates with the output of the zero-order TIA and an output. A first feedback resistance has one end that communicates with the input of the zero-order TIA and an opposite end that communicates with the output of the first opamp.
In other features, a capacitor has one end that communicates with the input of the zero-order TIA. The zero order TIA includes a second opamp having an input and an output. A third opamp has an input that communicates with the output of the second opamp and an output. A second feedback resistance has one end that communicates with the input of the third opamp and an opposite end that communicates with the output of the third opamp.
In yet other features, a fourth opamp has an input and an output that communicates with the input of the second opamp. A fifth opamp has an input that communicates with the output of the first opamp and an output. A third feedback resistance has one end that communicates with the input of the fourth opamp and an opposite end that communicates with the output of the fifth opamp.
In still other features, at least one higher order circuit is connected to the nested TIA circuit and includes an nth feedback resistance, an nth opamp, and an (n+1)th opamp.
In yet other features of the invention, a nested differential mode TIA circuit includes a zero-order differential mode TIA having first and second inputs and first and second outputs. A first differential mode opamp has first and second inputs that communicate with the first and second outputs of the zero-order differential mode TIA and first and second outputs. A first feedback resistance has one end that communicates with the first input of the zero-order differential mode TIA and an opposite end that communicates with the first output of the zero-order differential mode TIA. A second feedback resistance has one end that communicates with the second input of the zero-order differential mode TIA and an opposite end that communicates with the second output of the zero-order differential mode TIA.
In still other features, the zero order differential mode TIA includes a second differential mode opamp having first and second inputs and first and second outputs. A third differential mode opamp has first and second inputs that communicate with the first and second outputs of the second differential mode opamp and first and second outputs. A third feedback resistance has one end that communicates with the first input of the third differential mode opamp and an opposite end that communicates with the first output of the third differential mode opamp. A fourth feedback resistance has one end that communicates with the second input of the third differential mode opamp and an opposite end that communicates with the second output of the third differential mode opamp.
In still other features, at least one higher order circuit is connected to the nested TIA circuit and includes an nth feedback resistance, an (n+1)th feedback resistance, and an nth differential mode opamp.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.